1. Field of the Invention
The present invention relates to a motor driving circuit and a method thereof, and particularly relates to a motor driving circuit that can control a slew rate of a current flowing through a motor, and a method thereof.
2. Description of the Prior Art
FIG. 1 illustrates a prior art motor driving circuit 100. As shown in FIG. 1, the motor driving circuit 100 includes a control circuit 101, an H bridge circuit 103, an error amplifier 105, drivers 107 and 108, and a feedback resistor 109. The driving circuit 100 serves to control driving currents 11 and 12 flowing through the motor 111. The motor driving circuit 100 is configured to utilize a constant driving current, equal to voltage value of VREF divided by the resistance of the resistor 109, to drive the motor 111.
As shown in FIG. 1, the currents flowing through the motor 111 can have two paths, I1 and I2, to drive the motor 111 in two different rotation directions respectively. Since detailed operations of the motor driving circuit 100 are well known by persons skilled in the art, they are omitted for brevity here.
The motor 111 can be regarded as an inductance device plus a resistor, thus a voltage VM with a value
      V    M    =            L      ⁢                        ⅆ          I                          ⅆ          t                      +    IR  is generated across the circuit, wherein I is the driving current flowing through the motor, and R and L are respectively the inductance and resistance of the motor. At the moment that the control device 121 is turned on to drive the motor 111, the motor 111 will suffer a largest voltage across the motor 111, such that the motor 111 will experience a driving current with a high slew rate. The output voltage Vout1 at one terminal of the motor and a feedback voltage VFB are very close to a ground level. As time goes by, the current I1 flowing through the motor 111 gradually increases, and the current flowing through the control device 121 tends to the desired constant current accordingly. However, for some reasons, the current I1 flowing through the motor 111 may be higher than the current flowing through the control device 121, charging the parasitic capacitor located at the node connecting between the motor 111 and the control device 121, and causing the occurrence of high-frequency LC oscillation, such that the current I1 flowing through the motor 111 vibrates but converges to the desired constant driving current value, as shown in FIG. 2. The oscillation indicates the instability of the motor speed, even if the current 11 eventually converges to the constant current value. Therefore, such an issue should be improved.